If you read some of the article I linked to, the gist of it is that the smart alternator sees the leisure battery's voltage & tries to balance it out with the car's battery. I think a VSR is too simplistic ......
Yes, but I don't think he's 100% correct. The two batteries' states of charge will track each other if they are simply in parallel all the time, of course. With a VSR, it won't pull in while the car's alternator is allowing the vehicle battery to discharge, because the terminal voltage will sink too low, so the leisure battery won't discharge.
During periods of regenerative braking or when the car decides the battery has got a bit low, the voltage will rise, the relay will pull in and the leisure battery will receive charge. The car will probably monitor charging current as an additional indicator of the state of the battery, so it may well see the extra current drain into the leisure battery and decide extra charging is required.
Once it's happy with the state of charge of the battery, or the opportunity for regenerative charging has passed, the voltage will drop once again, the leisure battery will be disconnected and it will retain its' new charge level. I think it would work.
As an alternative, however, a charge controller that can boost the car's output voltage and inherently blocks any discharge path back from the leisure battery would also do the job.